Fibre optic sensor array reading device

ABSTRACT

A device for reading sensors in fiber optics which includes an integrated optics interferometer wherein the interference signal is slaved to a set-point value by means of a negative feedback signal. The measurement for reading the sensors is provided from the negative-feedback generated signal. In one of the disclosed embodiments generated delays of a value greater than an optical wavelength are measured by coupling the interferometer to a counting-up/counting-down system for interference fringe jumps.

This application is a Continuation of application Ser. No. 07/730,954,filed on Jul. 23, 1991, now abandoned.

The present invention relates to a fibre optic sensor reading device.

In order to decode the data coming from a network of coherentlymultiplexed sensors it is necessary to compensate the phase shifts withthe aid of an interferometric reading device. Fibre opticinterferometers of Mach-Zehnder type or interferometers of Michelsontype are used depending on the architecture of the sensor network. Inthe particular case of weakly coherent multiplexing, the use of nonbirefringent optical fibres is tricky given the accuracy required forthe adjusting of the length of the fibre segments used in passiveinterferometers of Mach-Zehnder type. In the general case, the Michelsoninterferometer is widely used in the laboratory given its greatversatility and ease of use. However, use outside the laboratory, forexample at an industrial site, is very tricky because, in particular, ofthe fragility of the movable mirrors of this interferometer.

The subject of the invention is a fibre optic sensor reading devicewhich is of small size, easy to regulate and to use, which can beemployed in a hostile environment (industrial surroundings, on-boardhardware etc.), which offers a sensitivity at least equal to that ofknown interferometers whilst retaining a good dynamic range, and whichhas the shortest possible response time.

The fibre optic sensor reading device according to the inventioncomprises an integrated optics interferometer with two arms of differentlengths, followed by a detector and by a circuit for slaving theinterferometer, the measuring device receiving the negative-feedbacksignal of the slaving circuit as the signal to be measured.

According to another characteristic of the invention, at least one ofthe arms of the interferometer comprises a phase modulator connected tothe slaving circuit. Thus, according to the invention, theinterferometer with two arms of different lengths permits compensationfor the average delay between the two wave trains produced from theoptical wave of a weakly coherent source, this delay being a function ofthe magnitude to be measured, the measurement consisting in determininga magnitude necessary for the slaving of the interferometer. Thus, themeasurement is carried out without mechanical displacement, resulting ina better sensitivity of the reader, whilst retaining a good dynamicrange, this reader being of short size and offering a short responsetime.

The present invention will be better understood on reading the detaileddescription of several embodiments, taken by way of non-limitingexamples and illustrated by the attached drawing in which:

FIG. 1 is a graph of a signal collected by a detector associated with acoherent sensor device and with a reader of the prior art.

FIG. 2 is a graph of a signal collected by a detector associated with acoherent sensor device and with a reader according to the invention,

FIG. 3 is a graph explaining the principle of operation for readingsignals with large delay, with the aid of a reader according to theinvention,

FIG. 4 is a block diagram of a reading device according to theinvention,

FIG. 5 is a block diagram of a variant of a part of the reading deviceof FIG. 4.

FIG. 6 is the simplified block diagram of a reading device according tothe invention for the reading of multiplexed signals,

FIGS. 8 and 10 are diagrams of interferometers used by the inventionand,

FIGS. 7, 9 and 11 are graphs of wave shapes used in the slaving circuitof the invention.

The principle of the measuring of physical magnitudes with the aid offibre optic sensors operating coherently will firstly be described.Operating coherently means that the datum relating to the physicalmagnitude to be measured is contained in the delay (that is to say thephase shift) between two optical wave trains generated from a singlewave produced by a weakly coherent source (coherence wavelength lessthan a few hundreds of microns).

A few orders of magnitude permit the advantage in coherence methods tobe specified in relation to the other techniques (modulation of theintensity of the optical wave, modulation of the delay between twopulses or of a frequency-modulated signal, etc.). The coherence length1_(c) can be as small as 30μm. With superluminescent diode sources, goodcoupling with monomode optical fibres can be obtained; coupled power ofthe order of 100μW.

From the point of view of the processing of the signal suchcharacteristics are similar to those which would be obtained with pulsesof 10⁻¹⁴ s and peak powers of 10⁸ W.

The principle of the measurement is as follows. For coherence sensors,the measurement can be carried out only by correlation. In fact, theoptical wave generated by a source with weak coherence is a randomsignal, this prohibiting carrying out the measurement with the aid offiltering techniques. This situation exhibits strong similarities withradars referred to as noise radars which, despite their very attractivetime-frequency ambiguity function, are in practice can (sic) used. Ingeneral the pulse compression technique which permits analysis byfiltering is very often preferred to that of noise radars.

If we denote A(t) the optical wave emitted by the source, the sensorgenerates a second wave A(t-τ) which differs from the first only by thedelay τ. The measuring of this delay (or phase shift) permits the valueof the physical magnitude (temperature, pressure, etc...) to bedetermined. At the output of the sensing part of the optical fibre, thetotal optical wave is

A_(T) (t)=A(t)+A(t-τ), and, by assumption, takes place in the case wherethis delay is much greater than the coherence time of the source τ_(c) :

    τ>τ.sub.c =λ.sub.c /c

where c is the speed of light.

From a practical point of view, this means that the average value over atime interval T (sufficiently large compared with the period of theoptical wave) of the product (correlation function): A(t).A(t-τ) is nil,or: ##EQU1##

In what follows this average value will be denoted by the symbolicnotation <>: ##EQU2##

A direct measurement of the wave A_(T) (t) does not therefore permitdetermination of τ. Two waves must be generated from A_(T) (t) so as tocompensate this delay τ before effecting interferences on a detector.Hence, from A_(T) (t) two new waves are generated A_(T1) (t) and A_(T2)(t) such that:

    A.sub.T1 (t)=k A.sub.T (t)=k' {A(t)+A(t-τ)}

    A.sub.T2 (t)=k A.sub.T (t-τ.sub.R)=k' {A(t-τ.sub.R)+A(t-τ-τ.sub.R)}

k and k' being proportionality factors.

Now if (τ-τ_(R)) is less than the coherence time, which will be calledmore simply "catching up the delay" the interference of A_(T1) (t) andA_(T2) (t) on a detector will contain a term proportional to<A(t).A(t-Δτ)>where Δτ=τ-τ_(R).

The signal which is observed on a detector as a function of τ_(R)therefore has the form represented in FIG. 1.

According to the invention an integrated optics interferometer (IOI) isused, the two arms of which exhibit a difference in length correspondingto a delay τ_(o) equal to the average delay τ_(m), τ_(m) being theaverage delay relative to the physical magnitude to be measured. Oncethis average delay is compensated, the interference signal is slaved toa set-point value, and the measurement is preferably carried out on thenegative-feedback signal after calibrating the reading device. Theslaving is effected by using one of the arms of the interferometer asphase modulator, this being easy a priori with an I.O.I.

We then have the situation illustrated by the graph of FIG. 2 whereτ_(CR) represents the negative-feedback delay introduced by the phasemodulator. Therefore: τ=τ_(m) +τ_(CR) knowing τ_(m) a priori and τ_(CR)by the voltage for controlling the negative-feedback signal, τ isthereby deduced, that is to say the delay due to the sensor and hencethe value of the physical magnitude to be measured. On the curve of FIG.2 the point A, exhibiting a delay τ_(CR) should be situatedsubstantially midway between the points B (minimum) and C (maximum) thatis to say substantially in the middle of a zone BC of large amplitude,this so as to obtain, during detection of the a.c. slaving modulatingsignal, an optimal signal over noise ratio. The slaving circuit isdescribed below.

The delay which can be induced with an integrated optical phasemodulator which is of the order of λ, the optical wavelength, has beenexamined. It is clear that situations exist in which the differencebetween the average delay τ_(m) (introduced by the difference in lengthbetween the two optical paths of the I.O.I.) and the delay τ (due to thesensing part of the optical fibre) can be greater than the wavelength.

According to the invention, the solution to this problem consists incoupling the system for negative-feedback of the slaving device to acounting-up/counting-down system which permits production of jumps fromone interference fringe to the next.

This principle is therefore as follows: if the delay τ_(CR) to begenerated is greater than a phase shift of π, a negative voltage jump iseffected corresponding to a phase shift of 2π and one unit is added tothe up-counter.

For the symmetrical situation in which the phase shift to be generatedis less than -π, this time a positive voltage Jump is produced(corresponding to a phase shift of 2π) and one unit is subtracted fromthe up-counter. The situation illustrated in FIG. 3 therefore occurs. Inthis figure, M_(c) represents the value of the up-counter and c thespeed of light (delay: λ/c<-> phase shift to be generated equal to 2π).

The orders of magnitude of the accuracy which can be achieved with thedevice of the invention will now be discussed.

The very simple case is considered of a silica temperature sensor:dn/dT≈10⁻⁵, the length of which generating the delay P is 1 cm. If it isassumed that the accuracy of the slaving and (sic) of the order of 10⁻³2π rd (this corresponding to a not very severe constraint), the accuracyin the temperature will be ##EQU3## if λ=1 μm, and P_(c) being thecoherence length of the source, the number of fringes present in thesignal will be P_(c) /λ. The dynamic range will therefore be, if 1_(c)≃30 μm ##EQU4## Which is in fact greatly superior to the technologicalpossibilities of fibre optics.

So as to slave the signal of the detector to a set-point value, it isadvantageous to operate on the derivative of the interference fringes.This means that a small alternating voltage: ε cos (ωt) must besuperimposed on the value of the control voltage which generates τ, andthe value of the signal with frequency 2(ω/2π) analysed with the aid ofa synchronous detection. This control voltage is a voltage which variesproportionally to the variation in the phase shift to be measured. Itsvariations are slow in comparison with the frequency of the modulatingsignal serving in the synchronous detection. The slaving is thus carriedout while seeking to annul this error signal.

Integrated optics interferometers have pass bands which can be of theorder of several GHz, and this measuring method can therefore permitanalysis of rapidly varying physical magnitudes.

This method does not permit an absolute measurement of the physicalmagnitude to be annulled. Errors can accumulate on theup-counter/down-counter, and it is necessary to continue with regularcalibrating in order to accurately redefine the zero of the counter.This problem is not particular to this measuring method, and there-calibrating to a reference signal is a solution well known per se.

An examplary embodiment of a reader according to the invention will nowbe described with reference to FIG. 4. An optical fibre 1 comprises asection 2 serving as sensor of physical magnitude. The fibre 1 is fed byan at least weakly coherent source 3, and is connected to a readingdevice 4 charged with analysing the delay τ produced in the sensor 2.According to the invention, the device 4 comprises an interferometer 5which has two arms 5A, 5B of different lengths, the am 5A being theshorter. Their difference in length can for example lie between 100 μmand 1 cm approximately.

According to a preferred embodiment, the interferometer 5 is of theMach-Zehnder type, in integrated optics. The interferometer 5 compriseson one of its two arms, for example the arm 5A, a phase modulator 6. Theoutput of the interferometer 5 is connected to a conventional detector7, itself connected to a synchronous detector 8. The output of thedetector 8 is connected in negative-feedback to the phase modulator 6.

According to the variant represented in FIG. 5, the reading device 4'comprises an interferometer 5', likewise in integrated optics, with twoarms 5'A, 5'B of unequal lengths, the arm 5'A being the shorter. On eachof the arms 5'A, 5'B is disposed a phase modulator, 6'A, 6'A (sic)respectively. The output of the interferometer 5' is connected to aconventional detector 7', itself connected to a synchronous detector 8'.A generator 9 of substantially sinusoidal voltage, producing a voltageof frequency f, is connected to a voltage input for referring thedetector 8' to the modulator 6'B. The output of the detector 8' isconnected to the modulator 6'A. The component of frequency 2f,demodulated, is collected on this output of the detector 8'.

A reading device for coherently multiplexed sensors has been representedin FIG. 6. A source 10 of light pulses is connected to an optical fibre11 in which a measuring device 12 is inserted. This device 12 comprisesn sensors which are either different sensors, or sections of sensors ofthe same optical fibre. The reading device 13 branched to the output ofthe device 12 is charged with measuring n delays of wave trainsgenerated by the n sensors. The reading device 13 comprises n shuntbranches 14.1 to 14.n from a main fibre 11' connected to the output ofthe device 12. Onto each of the branches 14.1 to 14.n is branched areading device 15.1 to 1 5.n (not represented in detail) such as thedevice 4 or the device 4'. These devices 15.1 to 15.n may each bepreceded by a polariser 16.1 to 16.n, each of the interferometers beingslaved in the manner represented in FIG. 4 or in FIG. 5. The principleof operation of coherently multiplexed sensors has been described forexample in French Patent Applications Nos 88 00780 and 88 00781.Obviously, the n integrated optics interferometers of the reading device13 can be produced on the same common substrate, during the samemanufacturing process.

The interferometer of the invention is advantageously produced accordingto the art for integrated optics circuits. According to a firstembodiment, it can be produced on a substrate such as LiNbO₃. The armsof the interferometer are optical wave guides formed by diffusion oftitanium.

According to another embodiment, a material comprising GalAs or GainAsPis used for the substrate of the integrated optics circuits. In thiscase, the optical waveguides are preferably formed epitaxially.

Given that the integrated optics circuits have very small dimensions(effective width of a Mach-Zehnder interferometer of approximately 100μm), a large number of them can be manufactured on the same substrate.

In order to slave the interferometer, a control voltage (which can beconsidered as a d.c. voltage C for a short lapse of time relative to theperiod of the signal which modulates it, and which is represented at thetop of FIG. 7) must be produced, on which is superimposed an a.c. signalV (represented in the middle of FIG. 7), which can be rectangular, butnot necessarily, this so as to generate a modulated error signal E, asrepresented at the bottom of FIG. 7. So as to simplify FIGS. 7, 9 and 11of the drawing, it has been assumed that in the very brief time intervalrepresented in these figures, the phase shift to be measured does notvary, and hence that the control voltage does not vary. Obviously, if alarge lapse of time is considered, this control voltage varies in linewith the variations in the phase shift to be measured, and exhibitsJumps in value on each occasion that the variations in the phase shiftexceed a 2π bracket (if, for example, the control voltage has itsnominal value for a phase shift lying between -π and +π, a first jump ofthis control voltage permits operation in the +M, +3M (sic) range, asecond in the +3π, +5π range, etc. and the same is true for negativevalues of phase shift: -π, -3π, etc. ), as is deduced from FIG. 3.

A first solution consists in applying the voltage C to one modulator(for example 6'A in FIG. 5) and the signal V to the other (6'B).

According to a second solution (FIG. 8), an interferometer 17 with twoarms 18, 19 is used. In one of the arms, for example, the arm 18, twomodulators 20, 21 are formed, and in the other, a single modulator 22.

An a.c. voltage of substantially rectangular shape V_(A) with nilaverage value, such as represented at A in FIG. 9, is applied to one ofthe modulators of the arm 18, for example the modulator 20. Asubstantially rectangular voltage VB such as represented at B in FIG. 9,the low level of which is equal to zero and the high level of which ispositive, is applied to the other modulator 21 of the arm 18. A voltageVC, such as represented at C in FIG. 8, which is synchronous with VB,the high level of which is equal to zero, and the low level of which isnegative and equal, in absolute value, to the high level of VB, isapplied to the modulator 22. The frequency of VB and of VC must be ahigher frequency than the maximum frequency of the variations in thephase shift to be measured. Subtraction of VB and of VC gives a positiveDC voltage of value equal to the amplitude of VB. This solution exhibitsthe advantage of avoiding the application of a d.c. voltage to one ofthe arms of the interferometer, since the d.c. voltage could causedamage to the integrated optics of this interferometer and provokeelectronic noise (noise inversely proportional to the frequency).

The third solution (FIG. 10) calls upon an interferometer 23 with twoarms 24, 25 each comprising a modulator 26, 27 respectively, whichinterferometer can be the same as the interferometer 5' of FIG. 5. Asubstantially rectangular signal with positive average value VD, such asrepresented at D in FIG. 10 (sic), is applied to one of the modulators,and to the other modulator a substantially rectangular signal withnegative average value synchronous with VD and such as represented at Ein FIG. 11. The low level of the signal VD is slightly negative, and itshigh level is positive and of greater value than the absolute value ofthe low level. The high level of the signal VE is slightly positive, andits low level is negative, of absolute value greater than the value ofthe high level. The difference VD minus VE gives the signal VF such asrepresented at F in FIG. 11. This signal VF shows up as a rectangularsignal superimposed on a positive d.c. component. This solutionexhibits, in addition to the advantages of the preceding solution, theadvantage of calling upon only digital circuits, and of requiring onlytwo phase modulators.

In these three solutions, if a lapse of time is considered which issufficiently long to be able to observe variations in the phase shift tobe measured, the control voltage is not a horizontal line, but a curvethe form of which reflects the form of the curve of the variations inthe phase shift. In order not to have to modify the shape of theabovementioned a.c. voltages (rectangular shape), their frequency mustbe markedly greater than the frequency of the variations in the phaseshift to be measured.

We claim:
 1. A device for reading sensors coherently, said devicecomprising:a single integrated optics interferometer having two arms ofdifferent length with at least one of said arms having a phasemodulator, a first detector, and circuit for slaving said interferometerincluding a feedback control synchronous detector for controlling saidslaving of said interferometer wherein a control voltage necessary forslaving said interferometer by a negative-feedback signal provides asignal to be measured in order to coherently read said sensors, whereinsaid phase modulator is integrated with said single interferometer andwherein said synchronous detector is directly connected to said phasemodulator.
 2. Device according to claim 1, wherein said slaving circuitcomprises a generator of substantially sinusoidal voltage, connected tothe interferometer and to a synchronous detector itself connected to theinterferometer.
 3. A device according to claim 1, wherein said slavingcircuit further includes a means responsive to delays to be measured ofa value greater than a given optical wavelength, for generating a jumpvoltage corresponding to interference fringe jumps.
 4. Device accordingto claim 1, wherein for reading n sensors disposed in series andmultiplexed coherently, said device further comprises, shunted onto amain fibre, n branches each comprising an interferometer and a detector.5. Device according to claim 3, wherein the interferometer of thereading device receives a first control voltage segment varying in linewith the variations in the delays to be measured and (C) controls theinterference fringe jumps, and an a.c. voltage segment (V).
 6. Deviceaccording to claim 5, wherein one arm of the interferometer comprises amodulator receiving a control voltage, and in that the other armcomprises a modulator receiving an a.c. voltage.
 7. Device according toclaim 1, wherein one arm of the interferometer comprises two modulatorsone of which receives an a.c. voltage with nil average value, and theother a first substantially rectangular signal, the other arm of theinterferometer receiving a second substantially rectangular signalsynchronous with the first, one of the substantially rectangular signalsbeing positive, and the other negative, their amplitude having equalabsolute values.
 8. Device according to claim 1, wherein one of the armsof the interferometer comprises a modulator receiving a firstsubstantially rectangular signal, the other arm comprising a modulatorreceiving a second substantially rectangular signal synchronous with thefirst, the average value of one of the signals being positive, and theaverage value of the other being negative.
 9. A device for readingsensors coherently, said device comprising:a single integrated opticsinterferometer having two arms of different lengths with at least one ofsaid arms having a phase modulator, a detector, and a circuit forslaving said interferometer including a feedback control means forcontrolling said slaving of said interferometer wherein a controlvoltage necessary for slaving said interferometer by a negative-feedbacksignal provides a signal to be measured in order to coherently read saidsensors, said slaving circuit further including a means responsive todelays of a value greater than a given optical wavelength, forgenerating a jump voltage corresponding to interference fringe jumps.